High-lift systems are utilized on wings of aircraft to increase lift or drag during take-offs and landings. One type of high-lift system includes flaps on trailing edges of the wings. The flaps are moveable control surfaces that may be extended during take-offs and landings, and retracted at cruising speeds.
A variety of actuation systems may be used to extend and retract trailing-edge flaps on a wing. One known type of actuation system includes two drive stations, with each drive station connected to an opposite side of the flap. In short, each flap is typically driven by a mechanical system from two locations on the flap.
A flap support couples a flap to a fixed portion of a wing. The flap support includes flap support ribs, an actuator, a link, a drive arm, and a carrier fitting. The actuator turns the drive arm, which moves the link, which, in turn, pushes or pulls on the carrier fitting. The carrier fitting rotates about a point relative to the fixed flap support ribs.
If the flap disconnects from one of the flap supports (for example, the link disconnects from the carrier fitting), the other flap support that remains coupled to the other end of the flap and the wing provides support for the entire flap plus air load. In this scenario, the flap experiences freewheeling skew. While the connected flap support is able to carry the resultant loads for a certain number of flights, the single flap support is unable to support the flap over an increased number of flights. That is, the single flap support is not designed to fully support the flap over numerous flights, as the single flap will wear at an increased rate and/or may be susceptible to failure due to fatigue.
Because the loss of both flap supports is potentially dangerous below a certain altitude, disconnected flap supports are to be detected as soon as possible; preferably within a single flight. In known aircraft, one or more position sensors are used to detect freewheeling skew. The position sensor(s) are used to detect flap deflections at a specific flight configuration and compared to the same deflection on the ground. If the difference between the two deflections exceeds a certain threshold, a skew alert is output, and the aircraft is then serviced during a maintenance operation.
With increased performance demands, airplane wings are becoming thinner and stiffer. Similarly, flaps are also becoming thinner and stiffer. For performance reasons, certain flaps may be so stiff that they do not sufficiently deflect so as to be detected by a position based sensor. As such, known methods of detecting skew using position based sensors may not be capable of detecting flap disconnects or freewheeling skew.
Other known methods of detecting flap skew directly measure the load in a drive mechanism, such as through strain gage based load cells. However, a strain gage based sensor is sensitive to unexpected and unaccounted loads, such as torsion introduced into a link due to friction. Also, strain gages are very small and include very small connecting wires, thereby rendering them relatively delicate when installed in a high vibration environment, such as a flap support which is located directly behind an aircraft engine. Further, strain gages are, by definition, fatigue machines. For instance, copper alloy strain gages may have less fatigue life (in cycles) than various parts to which they are attached. As such, strain gage based sensors may be too delicate and unreliable for use with flaps of wings.